Velocity-depth Trend Research Articles

Understanding the synergistic impact of stress release and cementation on sandstone using sound waves — Implications for exhumation estimation

Exhumation is the process that encompasses uplift and erosion, leading to the removal of overburden and the release of effective stress exerted on rocks. When estimating exhumation magnitude using the compaction trend method, it is commonly assumed that the physical properties of rocks are insensitive to stress reduction. However, recent laboratory evidence has indicated that porosity exhibits weaker sensitivity to stress release compared with velocity that can be significantly affected by stress release. This raises questions regarding the validity of irreversible compaction assumed by compaction trend method. It remains unclear whether the impact of stress release can be observed in real rocks in exhumed areas because there is a lack of methods to directly measure the impact of stress release on field data. In addition, studying real rocks is further complicated by the presence of rock diagenesis and its interaction with stress release. To address these knowledge gaps, this study uses stress-dependent burial and uplift modeling and interprets an extensive well-log data set using the modeling-derived evaluation metrics. Conceptual modeling suggests that methods that neglect the combined effect of cementation and stress release tend to underestimate the exhumation magnitude. Furthermore, we discover that the disparity between porosity sensitivity and velocity sensitivity to stress release can be leveraged to derive a metric we call “porosity inconsistency” that can serve as a qualitative and quantitative measure for identifying and evaluating stress release in sandstone using geophysical field measurements. We gather a significant amount of sonic velocity and porosity data from normally compacted and uplifted clean sandstones in the Norwegian Sea and the Barents Sea. Notably, we observe significant porosity inconsistency in the exhumed well 6510/2-1 in the Norwegian Sea. In the Barents Sea, which has experienced extensive Cenozoic exhumation, the well data reveal a varying pattern of porosity inconsistency increasing toward the north and decreasing toward the west. This distribution of porosity inconsistencies in the Barents Sea wells not only aligns with the spatial variation of exhumation reported in various studies but also exhibits a positive correlation with the magnitude of exhumation. Furthermore, the exhumation magnitude derived from velocity-depth trends is considerably lower than the magnitude obtained from porosity/density-depth trends for wells displaying significant porosity inconsistency. These observations provide support for the predictions made by the conceptual modeling. The results of this study enhance our understanding of the synergistic impact of stress release and cementation on sandstone. Moreover, these findings have implications for pore pressure prediction and core evaluation in exhumed areas. They also provide insights into the feasibility and interpretation of time-lapse data of reservoir injection, for which the effective stress is likely to decrease due to pore pressure buildup.

Estimate near-surface velocity with reversals using deep learning and full-waveform inversion: A field-data-driven method

Estimating shallow velocity from land seismic data is challenging due to complexities in the near surface. One such complexity arises from velocity reversals, which can be caused by anomalous high/low-velocity layer intrusions into the otherwise increasing velocity-depth trend. Velocity reversal imposes significant challenges for conventional velocity model building methods to accurately estimate near surface models. Turning ray tomography or tomostatics relies on accurate picking of first breaks, which becomes infeasible when velocity reversals generate shingling events. In this paper, we proposed a field-data-driven deep learning framework, which combines field-specific training, deep learning and full-waveform inversion, to estimate the near surface velocity model from land seismic data. Instead of training over purely random synthetic data, we propose to incorporate a-priori knowledge about the field under study to generate field-specific data set for training the proposed deep neural network. In addition, we combine the deep learning model with full-waveform inversion to further improve the near-surface velocity model. Synthetic and field data examples show that the proposed framework can successfully handle the velocity reversal complexity in land seismic data and achieve promising performance on estimating near surface velocity models.

New insights into tunnel valley locations and Cenozoic exhumation in the southwestern Baltic Sea based on Machine Learning aided seismic refraction tomography

The Cretaceous and Cenozoic evolution of the North German Basin is shaped by complex processes involving basin inversion, uplift and erosion, extension and several periods of Quaternary glaciations. Based on a densely spaced long-offset 2D seismic profile network covering the Bays of Kiel and Mecklenburg, we employ a Machine Learning algorithm to pick refracted first-arrival travel-times. These travel-times are used in a travel-time tomography to derive velocity models for the approximately upper 800 m depth of the subsurface. Investigating velocity-depth relations within the Upper Cretaceous strata and analyzing lateral velocity anomalies within shallow depths provide new insights into the magnitude of the Cenozoic basin exhumation and the locations of glacial tunnel valleys. Our findings suggest that previously observed bent-up structures in seismic images are caused by heterogeneous velocities in the overburden and do not represent actual reflectors. We provide strong indications that these misinterpretations of imaging artifacts are related to tunnel valleys even though these valleys might not always be resolvable in seismic reflection or sediment sub-bottom images. Comparing Upper Cretaceous velocity-depth trends to reference trends reveals significantly higher velocities in our study area. We interpret these differences as overcompaction and estimate the apparent Cenozoic exhumation in the Bay of Mecklenburg to be about 475 m. Within the Bay of Kiel, we observe an increase of the apparent exhumation from about 385 m (south) to about 480 m (north). Our study demonstrates the importance of near surface velocity analysis for the investigation of geological processes in shallow marine settings.

Seismic velocity-depth relation in a siliciclastic turbiditic foreland basin: A case study from the Central Adriatic Sea

Starting from reprocessed seismic profiles and borehole data, we investigate the Central Adriatic foredeep basin by deriving velocity depth trend of the Pliocene-Quaternary (PQ) siliciclastic succession (mainly composed by shales and sands). Relying on independent approaches to map two-way time (TWT) thickness of the PQ deposits, we converge on testing linear and exponential functions to predict VP depth trend. Results suggest that for large (>1500 m) thicknesses of the PQ deposits best fit is achieved by the exponential function VP(z) = c z(1-n) while for shallower and thinner deposits, a linear function like VP(z) = V0 + k z provides the best fitting estimates. We also investigate anomalies in velocity trend with depth and suggest that velocity drops observed within the PQ succession at significant depth (2500–3500 m) may reflect overpressured deposits. This hypothesis is consistent with the Apennine compression and high sedimentation rates in Central Adriatic during the Pliocene-Quaternary. Finally, we stress the importance of considering vertical-component phenomena and their time evolution when modeling foreland basins.

Applications of the trend-match attribute for interpreting seismic velocity data

A seismic velocity model is one of the products generated from a seismic imaging project. Recent advances in velocity modeling techniques have significantly improved the quality of seismic velocity data. Yet, the use of seismic velocity to guide geologic interpretations is still limited. This is mainly due to the overwhelming effect of compaction and the low-resolution nature of the seismic velocity model. Geologic boundaries and anomalies are often difficult to visualize from seismic velocity data alone. A new attribute called the trend-match attribute is proposed to reveal changes in velocity compaction trends from seismic velocity. The attribute is computed by comparing seismic velocity data with the regional velocity depth trends defined for different lithofacies using wells. We applied the trend-match attribute on several case studies to facilitate stratigraphic interpretation, horizon mapping, and erosion thickness estimation. Integration of the trend-match attribute volume with migrated seismic images can further constrain the geologic and stratigraphic interpretation at a regional scale.

Rock physics modelling based on depositional and burial history of Barents Sea sandstones

ABSTRACTUnderstanding how physical properties and seismic signatures of present day rocks are related to ancient geological processes is important for enhanced reservoir characterization. In this paper, we have studied this relationship for the Kobbe Formation sandstone in the Barents Sea. These rocks show anomalous low shear velocities and high VP/VS ratios, which does not agree well with conventional rock physics models for moderately to well consolidated sandstones. These sandstones have been buried relatively deeply and subsequently uplifted 1–2 km. We compared well log data of the Kobbe sandstone with velocity–depth trends modelled by integrating basin modelling principles and rock physics. We found that more accurate velocity predictions were obtained when first honouring mechanical and chemical compaction during burial, followed by generation of micro‐cracks during uplift. We suspect that these micro‐cracks are formed as overburden is eroded, leading to changes in the subsurface stress‐field. Moreover, the Kobbe Formation is typically heterogeneous and characterized by structural clays and mica that can reduce the rigidity of grain contacts. By accounting for depositional and burial history, our velocity predictions become more consistent with geophysical observables. Our approach yields more robust velocity predictions, which are important in prospect risking and net erosion estimates.

Compaction of quartz-kaolinite mixtures: The influence of the pore fluid composition on the development of their microstructure and elastic anisotropy

Shales play important roles in sedimentary basins, acting as both seals and reservoir rocks, and knowledge of their anisotropic velocity trends is of practical importance for correct seismic image processing and inversion, and for seismic to well tie. Whereas velocity-depth trends are extensively studied for conventional reservoirs and critical factors that control their compaction are well understood, little work has been done on shales. Compaction trends of shaly formations are controlled by a number of parameters such as clay mineralogy, silt fraction and depositional environment. This large number of parameters, which are difficult if not impossible to control in naturally deposited shales, make the study of shale compaction a statistically complex, multivariate and non-unique problem. In such a situation, laboratory experiments that allow precise planning of mixture mineralogy and chemical compositions of pore fluids as well as an accurate control of microstructural changes seems to be an attractive alternative.In this work, we have conducted two sets of compaction experiments on quartz-kaolinite mixtures with 100%, 75% and 60% of kaolinite powders (dry weight). In the first set, the use of a KCl brine solution to saturate the mixtures has led to the aggregation of the existing clay particles with each other and with silt particles. In the other set, the mixtures have been treated with a dispersant to separate existing clay aggregates into individual platelets. The mixtures have been further compacted in an oedometric cell at uniaxial stresses up to 30 MPa. Compressional (in vertical and horizontal directions) and shear (in vertical direction) ultrasonic wave velocities have been measured during these compaction experiments and the P-wave anisotropy has been estimated. The effect of the chemical composition of the pore fluid on shale microstructure has been studied using the micro-CT and SEM image analysis. Neutron diffraction experiments have been conducted to understand the orientation of clay platelets at the final stage of the compaction.Our research shows that the chemical composition of pore fluid significantly affects the microstructure of the clay component and via this the elastic properties of the artificial shales. The samples with the dispersed clay microstructure (DCM) have shown larger elastic anisotropies at the same porosity and significantly larger anisotropies at the same stress. The dispersed clay platelets have orientated preferably parallel to bedding plane while the clay platelets in the samples with the aggregated clay microstructure (ACM) show a wider angle distribution in the absence of quartz particles (100% kaolinite samples). In the case of 40% of quartz fraction though, the clay particles are more aligned in the ACM sample than in the DCM one.

Porosity and sonic velocity depth trends of Eocene chalk in Atlantic Ocean: Influence of effective stress and temperature

We aimed to relate changes in porosity and sonic velocity data, measured on water-saturated Eocene chalks from 36 Ocean Drilling Program drill sites in the Atlantic Ocean, to vertical effective stress and thermal maturity. We considered only chalk of Eocene age to avoid possible influence of geological age on chalk compaction trends. For each depth, vertical effective stresses as defined by Terzaghi and by Biot were calculated. We used bottom-hole temperature data to calculate the time–temperature index of thermal maturity (TTI) as defined by Lopatin. Porosity and compressional wave velocity data were correlated to vertical effective stresses and to TTI. Our porosity data showed a broader porosity trend in the mechanical compaction zone, and the onset of the formation of limestone at a shallower burial depth than the porosity data of the Ontong Java Plateau chalk show. Our porosity data do not show or at least it is difficult to define a clear pore-stiffening contact cementation trend as the Ontong Java Plateau chalk. Mechanical compaction is the principal cause of porosity reduction (at shallow depths) in the studied Eocene chalk, at least down to about 5 MPa Terzaghi׳s effective stress corresponding to a porosity of about 35%. This indicates that mechanical compaction is the principal agent of porosity reduction. Conversely, at deeper levels, porosity reduction is accompanied by a large increase in sonic velocity indicating pore-filling cementation. These deep changes are correlated with TTI. This indicates pore-filling cementation via an activation energy mechanism. We proposed a predictive equation for porosity reduction with burial stress. This equation is relevant for basin analysis and hydrocarbon exploration to predict porosity if sonic velocity data for subsurface chalk is available.

Compaction trends for shale and clean sandstone in shallow sediments, Gulf of Mexico

Compaction depth trends are important in drilling, basin modeling, and seismic exploration for several purposes: (1) to de-tect overpressure and hydrocarbon zones and distinguish them from seismic velocity anomalies; (2) to calculate interval velocities and depth conversion involving seismic data and Earth models; (3) to predict seismic signatures of sand-shale interfaces as a function of depth; and (4) to recognize over-compacted zones due to uplift. Several authors have studied the effects of compaction on the porosity of sands and shales (e.g., Magara, 1980; Ramm and Bjorlykke, 1994). The effects of compaction on velocity-depth trends have been provided by different authors (e.g., Al-Chalabi, 1997; Faust, 1951; Japsen, 2000). However, porosity and velocity depth trends in the shallow section are not well established. The main challenge in computing such trends is the paucity of well-log data in the shallow subsurface. Figure 1, a typical well log from the Gulf of Mexico, lacks measurements in the shall...

Constraints on velocity‐depth trends from rock physics models

ABSTRACTEstimates of depth, overpressure and amount of exhumation based on sonic data for a sedimentary formation rely on identification of a normal velocity–depth trend for the formation. Such trends describe how sonic velocity increases with depth in relatively homogeneous, brine‐saturated sedimentary formations as porosity is reduced during normal compaction (mechanical and chemical). Compaction is ‘normal’ when the fluid pressure is hydrostatic and the thickness of the overburden has not been reduced by exhumation. We suggest that normal porosity at the surface for a given lithology should be constrained by its critical porosity, i.e. the porosity limit above which a particular sediment exists only as a suspension. Consequently, normal velocity at the surface of unconsolidated sediments saturated with brine approaches the velocity of the sediment in suspension. Furthermore, porosity must approach zero at infinite depth, so the velocity approaches the matrix velocity of the rock and the velocity–depth gradient approaches zero. For sediments with initially good grain contact (when porosity is just below the critical porosity), the velocity gradient decreases with depth. By contrast, initially compliant sediments may have a maximum velocity gradient at some depth if we assume that porosity decreases exponentially with depth. We have used published velocity–porosity–depth relationships to formulate normal velocity–depth trends for consolidated sandstone with varying clay content and for marine shale dominated by smectite/illite. The first relationship is based on a modified Voigt trend (porosity scaled by critical porosity) and the second is based on a modified time‐average equation. Baselines for sandstone and shale in the North Sea agree with the established constraints and the shale trend can be applied to predict overpressure. A normal velocity–depth trend for a formation cannot be expressed from an arbitrary choice of mathematical functions and regression parameters, but should be considered as a physical model linked to the velocity–porosity transforms developed in rock physics.

Velocity-depth trends in Mesozoic and Cenozoic sediments from the Norwegian Shelf: Discussion

Storvoll et al. (2005) present log data from 60 wells on the Norwegian Shelf to investigate velocity-depth trends in sedimentary rocks. They present an interesting analysis of, e.g., the sonic-log velocities of overpressured Jurassic sediments and of source rock intervals. The authors do, however, reach the conclusion that “no general velocity-depth function can be used when performing more accurate analyses like depth conversion of seismic data, pore-pressure prediction, or basin modeling” (Storvoll et al., 2005, p. 359). This conclusion is surprising for two reasons: first, because it is not tested by the authors, and second, because the article presents very good evidence for the existence of normal velocity-depth trends for shale (i.e., functions that describe how sonic velocity increases with depth in relatively homogenous, sedimentary formations as porosity is reduced during normal compaction). The authors present log data from three areas along the Norwegian Shelf (but no examples of pressure versus depth): the northern North Sea, Haltenbanken, and the Barents Sea (about 60°N, 65°N, and 72°N, respectively). The main figures show velocity measurements versus depth for several wells in the three study areas (their figures 6, 8, 12). The authors compare the data with a linear velocity-depth trend: V p = 1477 + 0.57 × Z , which is similar to the normal velocity-depth trend for marine shale dominated by smectite-illite published by Japsen (2000): ![Formula][1] where V p is velocity in meters per second, and Z is depth in meters below seabed or ground level. This trend is almost identical to that developed by Scherbaum (1982) for Lower Jurassic shale in the Northwest German Basin and to that presented by Hansen (1996) based on data for Cretaceous–Cenozoic shales on the Norwegian Shelf (for depths less than about 2.5 km [1.6 mi]). It is interesting that the presented shale … [1]: /embed/graphic-1.gif

Velocity-depth trends in Mesozoic and Cenozoic sediments from the Norwegian Shelf: Reply

Sonic velocity, density, and resistivity log data from 60 wells on the Norwegian Shelf have been used to investigate velocity-depth trends in sedimentary rocks as a function of sediment composition, porosity, pore-pressure, burial-history, and compaction processes. A first-order linear velocity-depth trend line has been estimated from published velocity data. The data analyzed in this study, however, show significant variations from this trend line, indicating that no general velocity-depth function can be used when performing more accurate analyses like depth conversion of seismic data, pore-pressure prediction, or basin modeling. Lower Tertiary smectitic sediments from the northern North Sea and Haltenbanken are characterized by relatively low velocities compared to the overlying Pliocene and Pleistocene sediments, causing a distinct velocity inversion. A significant velocity increase at a burial depth corresponding to 70–100C was found and may reflect the alteration of smectite to illite and the initial precipitation of quartz cement in both sandstones and shales. Overpressured Jurassic sediments from Haltenbanken have lower velocities than equivalent hydrostatically pressured sequences but no significant porosity difference. The reduced velocities may be a direct response to lower effective stresses and, thus, reduced elastic compaction. Low velocities in source rocks are mainly attributed to the relatively soft kerogen and resulting velocity anisotropy. The high velocity/depth ratio of Barents Sea sediments (after correcting for Tertiary exhumation) is explained by the burial history of the area, the subsequent thermal exposure of the sediments over time, and thus, the amount of quartz cementation.

Velocity-depth trends in Mesozoic and Cenozoic sediments from the Norwegian Shelf: Discussion

Velocity-depth trends in Mesozoic and Cenozoic sediments from the Norwegian Shelf: Discussion

Velocity-depth trends in Mesozoic and Cenozoic sediments from the Norwegian Shelf

Velocity-depth trends in Mesozoic and Cenozoic sediments from the Norwegian Shelf

Modelling seismic response from North Sea chalk reservoirs resulting from changes in burial depth and fluid saturation

Changes in seismic response caused by variation in degree of compaction and fluid content in North Sea chalk reservoirs away from a wellbore are investigated by forward modelling. The investigated seismic response encompasses reflectivity, AVO and acoustic impedance. Synthetic seismic data are calculated on the basis of well data from the South-Arne and Dan fields, Danish North Sea and compared to field records. Seismic response predictions are based on three main tools: (1) saturation modelling, (2) compaction/decompaction modelling and (3) rock physics. Hydrocarbon saturation in North Sea chalk is strongly affected by capillary forces and transition zones in the order of 50 m are common. Advanced saturation height modelling is applied, which has proved robust for the prediction of saturation profiles in Danish chalk. Compaction modelling relies on exponential decay of porosity with depth, where abnormal fluid pressures are accounted for. A new set of compaction parameters is presented based on a normal velocity–depth trend and a velocity–porosity transform for North Sea chalk. The parameters appear to allow fairly precise predictions of abnormal fluid pressures from observed average porosity. Based on this, the relative contribution to porosity preservation by abnormal fluid pressure and early hydrocarbon invasion may be estimated. Rock physics theory is applied to obtain all necessary parameters for the complete set of elastic parameters for seismic modelling. Modelling results of importance in the search for subtle traps include: (1) correlation of reflectivity with porosity; (2) primary sensitivity of acoustic impedance is to porosity variation rather than hydrocarbon saturation; (3) the Poisson ratio is very sensitive to hydrocarbon saturation at high porosity, depending on fluid density contrasts. In addition, compaction modelling shows a clear effect of porosity preservation by hydrocarbons in the South-Arne Field, whereas this effect is negligible in the Dan Field. In both fields seismic signatures in field records originating from fluid changes are identified.

Estimating regional pore pressure distribution using 3D seismicvelocities in the Dutch Central North Sea Graben

The application of the empirical Eaton method to calibrated sonic well information and 3D seismic interval velocity data in the southeastern part of the Central North Sea Graben, using the Japsen (Glob. Planet. Change 24 (2000) 189) normal velocity-depth trend, resulted in the identification of an undercompacted and overpressured zone within the shaly Lower North Sea Group. Calculated overpressures near the bottom part of the group increase from east to west, from 3.5 to 5.5 MPa, that is in the direction of increasing sedimentary loading rates in Pliocene-Quaternary times. This ability of the application of sonic and 3D seismic velocity in empirical methods to assess the spatial distribution of overpressures in such shaly units is of importance because measured pressure data are usually not available for shaly sections.

Investigation of multi-phase erosion using reconstructed shale trends based on sonic data. Sole Pit axis, North Sea

Estimates from sonic data of removed overburden (burial anomalies) rely on identification of normal velocity–depth trends for relatively homogenous lithological units. In the North Sea, such trends are difficult to establish for Mesozoic formations that rarely are found at maximum burial and hydrostatic pressure. The analysis of erosional history based on sonic data can, however, be developed further where two homogeneous units are encountered in the same wells, because the two units may or may not have been simultaneously at maximum burial at a given location. If the two units were at maximum burial depth simultaneously prior to the most recent erosional event, the burial anomalies for the two units should be identical. If, however, the lower unit was at maximum burial before the upper unit, due to an intervening erosional event, the burial anomaly for the lower unit should exceed that of the upper unit. In the North Sea, Neogene erosion can be estimated from burial anomalies for the Upper Cretaceous–Danian Chalk Group. This allows corrections to be made of the depths of pre-Chalk formations to the situation prior to the onset of Neogene erosion. The normal trend for the pre-Chalk formation can now be traced more easily in a plot of velocity versus corrected depth, because the pre-Chalk formation was at maximum burial at more locations prior to Neogene erosion. This procedure is used to derive baselines for a Lower Jurassic shale unit, and for the Lower Triassic Bunter Shale and Bunter Sandstone using data from 123 British and Danish wells. The dominance of smectite/illite in the marine Lower Jurassic shale, and of kaolin in the continental Bunter Shale may explain why these two baselines diverge, and why those for the Bunter Shale and Bunter Sandstone converge. Burial anomalies for the Chalk Group and the Bunter Shale/Sandstone are calculated for 210 wells in the southwestern North Sea. Of these wells, 81 have data for both the Chalk and the Triassic level. The burial anomaly for the Triassic level exceeds that for the Chalk only along the Sole Pit axis. There, the Triassic deposits are suggested to have experienced maximum burial prior to an erosion event, in probably the Late Cretaceous or locally in the latest Jurassic. The erosion removed more than 2 km of mainly Jurassic–Lower Cretaceous or locally Triassic–Jurassic sediments. The burial anomalies for the Chalk and the Bunter levels are about identical over the rest of the southwestern North Sea Basin where they were both at maximum burial prior to Neogene erosion of up to 1 km of sediments of mainly Cenozoic age.

Overpressured Cenozoic shale mapped from velocity anomalies relative to a baseline for marine shale, North Sea

A study of interval velocities from almost a thousand wells reveals basinwide differences in physical properties of the Cenozoic deposits of the North Sea Basin. These differences relate primarily to the sediments below the mid-Miocene unconformity as testified by a subdivision of a subset of these wells. Velocity-depth anomalies are mapped relative to a constrained, normal velocity-depth trend derived for marine Jurassic shale: tt = 465.e (super -z/2435) +180, where tt is transit time in mu s m (super -1) , and z is depth in metres below sea bed. The upper Cenozoic deposits are close to normal compaction, whereas anomalies for the lower Cenozoic sediments outline a zone of undercompaction in the Central North Sea that corresponds to the overpressure in the Upper Cretaceous-Danian Chalk. The overpressure results from a balance between the load of the upper Cenozoic deposits, and the draining determined by the thickness and sealing quality of the lower Cenozoic sediments. The shale trend may be more widely applicable to marine shale dominated by smectite/illite. This suggestion is based on the observed correspondence between velocity anomalies and pressure data, and due to the match between trends for marine shale of different ages in the North Sea and in the US Gulf Coast area over a significant velocity range.

Regional Velocity-Depth Anomalies, North Sea Chalk: A Record of Overpressure and Neogene Uplift and Erosion

A normal velocity-depth trend for the Upper Cretaceous-Danian Chalk Group is determined by identifying interval-velocity data that represent maximum burial in areas unaffected by overpressuring; these data are derived from 845 wells throughout the North Sea Basin. Data from pelagic carbonate deposits on a stable plateau constrain the trend for shallow depths. Positive velocity anomalies relative to the trend are mapped along the western and eastern margins of the North Sea Basin, and reflect regional Neogene uplift and erosion of up to 1 km along the present-day limit of the Chalk. A hiatus at the base of the Quaternary increases in magnitude away from the basin center, where a complete Cenozoic succession is found. This hiatus is consistent in size with the missing section estimated from Chalk velocities when allowance is made for the Quaternary reburial of the Chalk. Negative velocity anomalies in the central and southern parts of the basin outline an area within which overpressures in the Chalk exceed 10 MPa, equivalent to a burial anomaly greater than 1 km relative to the normal trend. The Chalk pressure system is primarily dependent on overburden properties because retention of overpressure generated by the load of the upper overburden depends on the thickness and sealing quality of the lower overburden; therefore, the Chalk is considered to represent a regional aquitard, and the hydrodynamic model of long-distance migration within the Chalk is rejected. The Neogene uplift and erosion of the margins of the North Sea Basin and the rapid, late Cenozoic subsidence of its center fit into a pattern of late Cenozoic vertical movements around the North Atlantic.

Quantification of Tertiary erosion in the Inner Moray Firth using sonic velocity data from the Chalk and the Kimmeridge Clay

Sonic velocities for the Chalk and Kimmeridge Clay were analysed to determine the importance and magnitude of Tertiary erosion for 26 wells in the Inner Moray Firth. Apparent erosion (height above maximum burial depth) was derived by the displacement, along the depth axis, of a given velocity-depth trend from the normal (undisturbed) trend. The similarity in apparent erosion results derived from the Cretaceous Chalk and Late Jurassic Kimmeridge Clay successions of the Inner Moray Firth suggests that burial beyond present depths caused the observed overcompaction rather than any sedimentological and/or diagenetic process. Resultant apparent erosion estimates are variable across the Inner Moray Firth. Such values reach approximately 1 km in the western part of the basin and decrease progressively to zero in the eastern Inner Moray Firth. However, the actual magnitude of erosion (equal to the sum of apparent erosion and post-erosional Tertiary burial) was approximately 1 km throughout the whole Inner Moray Firth area, indicating that Early Tertiary erosion was regional and not simply restricted to the inverted basin margins. However, a well defined mechanism sufficient to have generated the regional event is still sought.